Question

What is the structure of the dye the following chemical reaction?

A)

B)

C)

D)

Answer 1

A reaction of the diazonium salt with the electron-rich and highly reactive aromatic compounds to form azo compounds is known as the coupling reaction. Anthranilic acid is converted into the diazonium salt of anthranilic acid.in the next step diazonium salt undergoes a coupling reaction with N, N –Dimethyl-4-aminophenyl carboxylic acid.

Complete step by step answer:
Benzene diazonium salt reacts with highly reactive (i.e. electron rich) aromatic compounds such as phenol and amine to form azo compounds, $\text{ Ar}-\text{N = N}-\text{Ar }$. The reaction is known as the coupling reaction. These azo compounds are generally coloured and are used as a dye. The colour of azo compound is due to an extended conjugate system involving double bonds of both the aromatic rings through $\text{ }-\text{N = N}-\text{ }$ the double bond.
The coupling reaction is an example of an electrophilic substitution reaction in which the diazonium cation with a positive charge on the terminal nitrogen acts as an electrophile and the electron-rich compounds such as phenol or the amine acts as a nucleophile.
Here the reaction of the formation of colour dye proceeds via two steps.

Step 1) Preparation of diazonium salt:
Diazonium salt of anthranilic acid is prepared by heating an ice-cold solution of anthranilic acid in excess of the mineral acid such as $HCl$ with an ice old solution of sodium nitrite dissolved in water. This process is known as the diazotization reaction. The generation of diazonium salt of anthranilic acid is as shown below:

Step 2) Coupling reaction:

In the next step, the diazonium salt of anthranilic acid (obtained in step 1) is treated with the N, N –Dimethylaniline. The N, N –Dimethylaniline is amine .the diazonium cation has a positive charge on the terminal nitrogen acts as electrophile .the N, N –Dimethylaniline loses its hydrogen from the para position such that the electrophile attacks on the negative charge of amine. This results in the formation of a coupled product (2-N, N –dimethyl-4-aminophenyl) azobenzene carboxylic acid. The reaction is as shown below:

(2-N, N –dimethyl-4-aminophenyl) azobenzene carboxylic acid is a red colour azo dye.

Therefore, option (B) is correct.

Note: (2-N, N –dimethyl-4-aminophenyl) azobenzene carboxylic acid is commercially known as methyl red. It has extended conjugation. It is a red colour dye in a protonated form. It is a red colour acid-base indicator. Under the $\text{ pH }$ value of $\text{ 4}\text{.4 }$ is red and yellow above the $\text{ pH }$ value of $\text{ 6}\text{.2 }$. It has wide application as an acid-base indicator.